A motor vehicle is provided with an evaporated fuel suppressing apparatus (canister) for reducing the release of an evaporated fuel into the atmosphere from a fuel tank of a gasoline internal combustion engine. In general, an adsorbent such as activated carbon is filled into the apparatus, thereby the evaporated fuel is adsorbed and trapped, and during the running of an engine, air is introduced into the canister to desorb the adsorbed evaporated fuel for combusting the desorbed fuel in the engine. In the United States, a large-sized canister (On Board Refueling Vapor Recovery, ORVR) is equipped to trap the gasoline vaporized in oil supply.
In recent years with the increased worldwide attention focused on improvements in the ambient environment and global warming prevention, environmental measures to be required for motor vehicles are getting severer and severer. The United States has LEVII and PZEV as regulations concerning fuel vapor to be discharged from motor vehicles. In the future, these regulations will be strengthened; the LEVII will become LEVITT (almost equivalent to PZEV), which will be introduced and enforced in stages in California from 2014.
In order to accomplish such strict environmental regulations, a canister suitable for fuel motor vehicles, an activated carbon to be filled (or packed) in the canister, and a combination of activated carbons in a canister are under investigation.
Japanese Patent Application Laid-Open Publication No. 2005-510654 (JP-2005-510654A, Patent Document 1) discloses a method for reducing fuel vapor emissions comprising the steps of contacting the fuel vapor with an initial adsorbent and a subsequent adsorbent, the initial adsorbent having incremental adsorption capacity at 25° C. of greater than 35 g n-butane/L between vapor concentrations of 5 vol % and 50 vol % n-butane, and the subsequent adsorbent having an incremental adsorption capacity of less than 35 g n-butane/L between vapor concentrations of 5 vol % and 50 vol % n-butane. According to this document, the low-emission performance is achieved by use of an activated carbon having a general high adsorption performance as the initial adsorbent disposed in the fuel tank side of the canister, and use of an adsorbent having a constant adsorption performance over wide fuel vapor concentrations, not having a high adsorption performance but having a high desorption performance as the subsequent adsorbent disposed in the discharge side. Moreover, this document also discloses the subsequent adsorbent is volumetrically diluted with a non-adsorbing filler.
Japanese Patent Application Laid-Open Publication No. 2009-19572 (JP-2009-19572A, Patent Document 2) discloses that the outside leakage of fuel vapor through an exhaust port of a canister is suppressed, wherein an adsorbent layer located near a port for introducing the fuel vapor comprises an adsorbent having a microporous structure having a pore size ranging from not smaller than 1 nm to smaller than 100 nm, an adsorbent layer located near the exhaust port comprises an adsorbent having the microporous structure and a macroporous structure having a pore size of not smaller than 100 nm. Further, this document discloses combination use of a small granular adsorbent (a formed activated carbon or a pulverized activated carbon) having a microporous structure and a large granular adsorbent (a formed activated carbon) having a particle diameter larger than that of the small granular adsorbent and having a microporous structure and a macroporous structure, and discloses that the large granular adsorbent is obtained by mixing a powdery activated carbon, at least one selected from the group consisting of powdery bentonite, silica sol and alumina sol, serving as a binder, a powdery meltable core material which is able to evaporate with the burning, and water, forming the resulting mixture into a cylindrical shape, and burning the formed mixture after drying.
The canisters described in these documents, however, sometimes deteriorate the low-emission performance when the loading amount to the canisters (an amount of a gasoline component evaporated from a gasoline tank during stopping the motor vehicle) is larger.
In these days, hybrid vehicles, which have a combination of gasoline engine and electric motor, are becoming the mainstream. A hybrid vehicle is confronted with an increase of a loading amount to a canister, since the amount of purge air, which is used for desorbing an adsorbed fuel component in a canister to introduce the evaporated fuel component into an engine as a fuel, is significantly decreased. Even in the case of such a large loading amount to the canister, the achievement of low-emission performance is expected. It is therefore necessary to show both performances, a capacity for trapping the evaporated fuel and a capacity for desorbing the evaporated fuel.
WO2009/031467 A1 (Patent Document 3) discloses use of an adsorbent in which an integrated volume of pores having an average diameter of 3,000 to 100,000 nm as measured with a mercury porosimeter is 6.5 mL/dl or more and an equilibrium adsorption as measured in a n-butane volume concentration of 2,000 ppm is 0.16 g/dl or more, for adsorbing an evaporated fuel gas. This document discloses a process for producing the adsorbent, in which an activated carbon, a smoothing agent, an inorganic compound soluble in an acid (e.g., calcium carbonate), an aggregate resistant against an acid, a binder, and water are mixed and granulated, and the resultant is dried, subjected to particle size regulation, washed with an acid for dissolving the inorganic compound, washed with water, and dried. This document also discloses that, as an example, the total amount of the aggregate and calcium carbonate is not less than 75% by weight relative to 100 parts by weight of the activated carbon. According to this document, however, it is necessary to use the aggregate and the acid-soluble inorganic compound in large quantities, and the adsorbent is still insufficiently efficient in trapping or adsorbing an evaporated fuel gas.